The present invention relates to an improved method of compensating for fading distortion in a digital mobile communication system and a circuit thereof.
As a conventional fading compensation technique in a digital mobile communication system, there is known a system described in, for example, "RAYLEIGH FADING COMPENSATION METHOD FOR MULTI-LEVEL QAM FOR LAND MOBILE COMMUNICATIONS", Review of The Communications Research Laboratory, Japan, February 1991, pp. 87-98 (particularly, FIGS. 2 to 7) and "PERFORMANCE OF IMPLEMENTED 16 QAM MODEM FOR LAND MOBILE COMMUNICATION SYSTEMS", Review of The Communications Research Laboratory, February 1991, pp. 99-107 (particularly FIG. 4). "MULTI-LEVEL QUADRATURE AMPLITUDE MODULATION TECHNIQUES", Review of The Communications Research Laboratory, February 1991, pp. 85-86 describe abstracts of the aforementioned two papers. When a multi-level QAM (Quadrature Amplitude Modulation) wave is transmitted under Rayleigh fading, an amplitude and a phase thereof are varied due to the fading to thereby change a magnitude and an inclination of a signal space diagram thereof while the whole form or configuration of the signal space diagram is not changed. The above system utilizes the characteristic that the whole configuration of the signal space diagram is not changed. On a transmitting side, transmission data of a frame format having a predetermined number of symbols formed by periodically inserting a predetermined symbol (hereinafter referred to as a pilot symbol) of the signal space diagram into a series of data symbols is transmitted and on a receiving side a signal space position of the pilot symbol is detected and an amount of fading distortion of each symbol is estimated and compensated on the basis of relative positional relation of points to decode the data.
FIG. 3 shows an example of the pilot symbol in 16 QAM in which a point A of 16 symbols is used as the pilot symbol.
An example of a prior art adopting the above system is now described with reference to FIG. 2.
In FIG. 2, numeral 2 denotes a clock reproducing unit, 3 a data symbol input control unit, 4 a data delay unit, 5 a fading distortion amount calculation unit, 6 a fading distortion estimation and compensation unit, 9 a pilot symbol input control unit, and 10 a pilot detection unit.
An input signal is a received and detected complex baseband signal u(t). The received signal is supplied to the pilot detection unit 10 and the clock reproducing unit 2 so that a predetermined pilot detection signal and clock signal are produced by the units 10 and 2, respectively. The pilot symbol input control unit 9 takes out only the pilot symbol in accordance with the pilot detection signal to supply the pilot symbol to the fading distortion amount calculation unit 5. When a transmission complex baseband signal is z(t) and a fading distortion is c(t), respectively, and noise and other influence are neglected, the received complex baseband signal u(t) is expressed by EQU u(t)=z(t).times.c(t) (1)
Accordingly, when a reception pilot symbol is up and a transmission pilot symbol is p, an amount of fading distortion c.sub.p in the pilot symbol is calculated by EQU c.sub.p =u.sub.p /P (2)
The fading distortion amount calculation unit 5 calculates the fading distortion amount in the pilot symbol in accordance with the equation (2).
The received data is supplied to the data symbol input control unit 3 which takes out only a data symbol at a data identification point from the received data and the data symbol is supplied to the data delay unit 4. The data delay unit 4 delays the data symbol by a time required to estimate the fading distortion amount in the data symbol in the fading distortion estimation and compensation unit 6.
The fading distortion estimation and compensation unit 6 estimates the fading distortion amount in the data symbol on the basis of the fading distortion amount of the pilot symbol calculated by the fading distortion amount calculation unit 5 by means of the interpolation method as shown in FIG. 5. Further, the fading distortion estimation and compensation unit 6 multiplies an inverse number (conjugate complex number) of the calculated distortion amount by the data symbol inputted from the data delay unit 4 to compensate the fading.
FIG. 4 shows an example of a frame format used in the prior art. This frame format is described in the aforementioned "Review of The Communications Research Laboratory", February 1991, page 89.